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A symbol representing cardinality or number signs in set theory and numerical contexts.

\# image

Examples

Cardinality or size of a set

\#{A} = 5 \text{ means set A has 5 elements}

Number of elements notation

\text{The number of solutions is } \#{x \in \mathbb{R} : x^2 = 4} = 2

Counting elements in an index set

\#{i \in I : a_i \neq 0} < \infty